1. Field of the Invention
The invention relates to methods of holding an article and, more particularly, to methods of holding semiconductor chips with grooved magnetic carriers.
2. Description of the Prior Art
In the manufacture of beam-lead semiconductor devices, wafers (slices) of semiconductor material are subjected to numerous photoresist, mask, etch, diffusion and metallization operations which form a large number of the devices in the wafer. The wafers are made fairly thick, about 0.020 inches, in order to withstand handling during this processing. Further, the devices are generally formed with their leads interleaved in order to obtain the maximum number of devices from the wafer. This precludes scribing and breaking the wafer into separate devices. Instead, the wafer is cemented to a support, often a sapphire disc, with wax or polypropylene; thinned to about 0.002 inches by lapping; masked to protect the bodies of the devices; and then selectively etched to yield an array of separate devices still adhered to the support.
The devices must be kept in the precise array and orientation in which they were formed for accurate bonding to the circuits in which they will be used but the adhesive must be removed so that it will not interfere with the bonding.
Jannett et al., U.S. Pat. No. 3,681,139 solve this problem by placing a magnetic grid over the devices, to maintain their orientation, and supporting them on a porous or perforated magnetic block having mesas. The adhesive is dissolved while the devices are retained by the magnetic grid. The solvent flows between the mesas to remove adhesive adhering to the underside of the devices. However, in order to remove the grid to get at the devices without losing their orientation and position, vacuum must be applied through the perforations or porous support. That is, from this point on until they are bonded to their circuits, the devices must be held to the support by vacuum.
The inconvenience of the grid frame and the vacuum may be overcome by the methods of Gruszka, U.S. Pat. No. 3,809,233 or Hughes, Jr. et al., U.S. Pat. No. 3,783,499 which use a magnetized support or carrier plate in conjunction with a magnetic device. But the surface of the support is not channeled. As a result the solvent cannot readily flow beneath the devices to remove the adhesive without moving or shifting the devices. Gruszka adds a porous magnetic cover to prevent shifting and Hughes et al. do not mention solvent removal of the adhesive.